1. Field of the Invention
The present invention relates to a polarization independent silicon directional coupler and to various optical devices, such as a Mach-Zehnder interferometer and a ring resonator, that incorporate the directional coupler.
2. Description of the Related Art
In optical subscriber systems, since upstream communication (optical transmission from subscribers to the central office) and downstream communication (optical transmission from the central office to the subscribers) are carried on a single optical fiber, the upstream and downstream optical signals have different wavelengths. The central office and the subscriber equipment must therefore have optical wavelength filters for the corresponding wavelengths. In general, the optical subscriber equipment, referred to as an optical network unit (ONU), includes a light emitting element and a photodetector having optical axes that must be aligned through respective wavelength filters with the optical axes of an optical splitter/combiner when the equipment is assembled.
Recently, waveguide-type optical wavelength filters, which obviate the need for optical axis alignment, have been under study. Some known optical wavelength filters of this type use Mach-Zehnder interferometers, directional couplers, and grating reflectors, as disclosed in U.S. Pat. Nos. 4,860,294, 5,764,826, 5,960,135, and 7,072,541 and Japanese Patent Application Publication No. H8-163028.
Optical wavelength filters using Mach-Zehnder interferometers as described in the above patent documents have the advantage that their wavelength characteristics can be designed on the basis of optical circuit theory. The silicon Mach-Zehnder optical wavelength filters used in ONUs have the disadvantage, however, that their effective refractive index and coupling coefficient exhibit strong wavelength dependency, and there is also a polarization dependency that cannot be eliminated without special measures, such as varying the waveguide width in the directional coupler.
The inventor has found that polarization independent wavelength separation in the ONU wavelength band can be achieved by optimizing the dimensions of the two optical waveguides forming the directional coupler, including the distance between their central axes, with respect to the refractive index of the clad.